fix: resolve all ruff lint errors and type checker warnings

- Use Annotated[str, Form()] for FastAPI dependencies (FAST002)
- Add missing type annotations across src/ and tests/ (ANN001/003/201/202)
- Reduce function arguments via request.form() reads (PLR0913)
- Combine return paths to reduce return statements (PLR0911)
- Use anyio.Path for async-safe filesystem operations (ASYNC240)
- Extract constants, helpers, and dict comprehensions for clarity
- Move inline imports to top-level (PLC0415)
- Use raw strings for regex match patterns (RUF043)
- Fix redundant get_session_user call in delete_user (not-iterable)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/test_invite/test_service.py

@@ -1,3 +1,4 @@
+from collections.abc import AsyncIterator
 from datetime import UTC, datetime, timedelta
 from pathlib import Path
 
@@ -14,7 +15,7 @@ MIGRATIONS_DIR = (
 
 
 @pytest.fixture
-async def db():
+async def db() -> AsyncIterator[aiosqlite.Connection]:
     conn = await aiosqlite.connect(":memory:")
     conn.row_factory = aiosqlite.Row
     await conn.execute("PRAGMA foreign_keys=ON")